1. Field of the Invention
The field of the invention relates to a gas analyzer for measuring at least two components of a gas. The gas analyzer comprises an emitter for being able to emit infrared radiation through the gas, a filter assembly for allowing a transmission of predetermined wavelengths emitted by the emitter, and a detector for receiving wavelengths emitted by the emitter and penetrated through the filter assembly.
2. Description of Related Art
In anesthesia or in intensive care, the condition of a patient is often monitored e.g. by analyzing the gas exhaled by the patient for its content. For this reason either a small portion of the respiratory gas is delivered to a gas analyzer or the gas analyzer is directly connected to the respiratory circuit. In a non-dispersive infrared (NDIR) gas analyzer the measurement is based on the absorption of infrared (IR) radiation in the gas sample. A radiation source directs a beam of infrared radiation through a measuring chamber to a radiation detector whose output signal depends on the strength of the absorption of the radiation in the sample gas. The optical wavelength band used for the measurement is selected without dispersive elements such as a prism or a grating using an optical bandpass filter. The radiation source typically comprises an electrically heated filament or surface area and radiation collecting optics and emits radiation within a broad spectral region. The gas sample to be analyzed, i.e. the sample gas, is fed through the measuring chamber, whereupon the gas mixture is included in the chamber for analysis. The measuring chamber can be a tubular space provided with entrance and exit windows that are transparent at the measurement wavelength and with inlet and outlet for the sample gas. Radiation is absorbed by the gas sample when passing through the measuring chamber. The radiation detector generates an electrical signal that depends on the radiation power falling on its sensitive area. The detector type in a gas analyzer depends on its measurement wavelength. For measurement within a broad spectral range a thermal detector is convenient because its sensitivity only depends on the efficiency of the conversion of radiation to heat. To make the detector's output signal sensitive to a certain gas component, the wavelength band of the radiation coming to the detector is selected so that the gas component absorbs radiation within it. This selection is made using an optical bandpass filter whose bandwidth is typically 1%-2% of the center wavelength.
In NDIR multigas analyzers, the absorption of the gas sample is measured at several wavelength bands, selected to match the absorption spectra of the gas components of interest. This can be accomplished by using one radiation detector and by changing the optical bandpass filters on the optical path in succession. It is also possible to use several radiation detectors, combined with their corresponding bandpass filters. In addition to these measurement detectors, there may be one or more reference detectors. The reference detectors typically receive radiation from the radiation source at wavelength bands where the sample gas is known to have no or little absorption. To measure the strength of absorption, it is necessary to know the zero levels of the analyzer at the measure wavelengths. The zero level is the detector signal obtained at a wavelength when the sample gas does not absorb IR-radiation at that wavelength. The strength of absorption is calculated by forming the ratio between the zero level signal and the detector signal, supposing that absence of radiation results in a zero or otherwise known signal. It is possible to update the zero levels by separately measuring zero gas that is known to not absorb radiation at the measurement wavelengths. This method is commonly used in a sidestream configuration, where a gas sample is drawn from the respiratory circuit and analyzed separately.
It is also possible to obtain estimates for the zero levels without zeroing the analyzer with gas. This can be accomplished by the use of reference filters, whereupon the detector signals are measured at reference wavelengths where the gas sample is known not to absorb IR radiation. It is also possible to use separate reference detectors together with reference filters and use the output signals of the reference detectors as estimates for the zero levels at the measurement wavelengths. These estimates are continuously available together with the detector signals obtained at the measurement wavelengths. It is often sufficient to use one or two common reference wavelengths for all measurement wavelengths, especially if the measurement wavelengths are close to each other. The reference wavelengths can also be chosen is such a way that they can compensate for disturbing matters like water in liquid or gas form. This method is commonly used in a mainstream configuration where the analyzer is positioned to measure across the respiratory tube.
In the clinically used gas analyzer of mainstream type the whole volume or at least the main portion of the breathing air or gas mixture flows through the analyzer and its measuring chamber. Because the measuring chamber is in the breathing circuit, it is easily contaminated by mucus or condensed water. Thus, it is necessary to use one or more reference wavelengths in a mainstream analyzer in order to have good enough estimate for the zero level continuously available.
A clinical mainstream gas analyzer must be small, light, accurate and reliable. It is not possible to zero it during its normal operation. Yet, the analyzer must maintain its accuracy even if the measuring chamber would be contaminated. Due to these requirements, only single gas analyzers for carbon dioxide CO2 have been available and no really compact multigas analyzers of the mainstream type have been commercially available. The best construction would be a single path analyzer because then e.g. contamination of the measuring path would influence both measuring and reference wavelengths similarly and the effect on the gas concentration value would be eliminated. However, this is difficult to accomplish using a multiple of discrete bandpass filters. Either it would require a rotating filter wheel, which may be big and apt to mechanical damage, or a number of beam splitters to separate the different bandpass wavelengths to different detectors. Only the former method can make use of a single detector and thus avoid the problem of differences between separate detectors. However, with discrete dielectric optical bandpass filters it does not seem to be possible to make an analyzer small enough to apply also for patients like small children and neonates.
It is an additional problem that the different respiratory gases have so widely spaced wavelength regions of absorption. Carbon dioxide and nitrous oxide can be measured between 3900 nm and 4600 nm whereas all anesthetic agents absorb in the 8000 nm to 10000 nm region. The wavelength change of the transmission band of a dielectric IR filter with angle of incidence is, e.g., far too small to cover both ends of the wavelength region of interest. Therefore, such a single path solution would not be applicable.
Still another requirement is that the measurement has to be fast enough to measure the breathing curve. In practice, the rise time would have to be in the order of 200 ms. An interferometric analyzer for detection of substances can be a Fabry-Perot type interferometer with possibility of electrical modulation. Often the mirrors are closely spaced, but the distance is still several wavelengths. The free spectral range (FSR) may be only about 10 nm and the whole spectral region is covered by a large number of very narrow transmission peaks 10 nm apart. The free spectral range is the frequency or wavelength space between consecutive transmission peaks in the transmission spectrum of a Fabry-Perot interferometer. The SFR is inversely proportional to the distance between the reflective surfaces in the interferometer. The region of interest in this case must be chosen using discrete bandpass filters on a wheel. Thus, this solution cannot be made more compact or faster than an analyzer using only a filter wheel to select the measurement wavelengths.
A single path gas analyzer using a micromechanical and electrically tunable Fabry-Perot interferometer is well-known in the art. Contrary to the previous solution the resonator is very short, even as short as only half the wavelength of interest. This broadens the free spectral range of the interferometer. The analyzer can be made very compact but the construction allows wavelength tuning only within about 10% of a chosen wavelength. This makes measurements of multiple gases almost impossible unless the wavelength regions are close together.
Within the near infrared wavelength region up to about 1700 nm a solution with Fabry-Perot tunable filters in series is also known. A superluminescent light emitting diode is preferably used as source and either source and filters or filters and detector are integrated on an optical bench within a hermetic package. The use is primarily optical telecommunication. The filters are micromechanical with a free spectral range of about 200 nm, meaning that the space between the mirrors of the Fabry-Perot filter is a multitude (>3) of half the design wavelength.